Since the piezoelectric effect (piezoelectricity) was discovered in quartz crystals about 100 years ago by Jacques and Pierre Curie, it has been used in various fields, including medical, military, household electric appliance, and exploration fields. Particularly, since a piezoelectric ceramic material was developed around World War II, technologies that use the same have been widely developed, including accelerometer sensors, IR sensors, ultrasonic transducers, speakers, microphones, actuators, sonars, and the like.
The piezoelectric effects are classified into direct piezoelectric effect and converse piezoelectric effect. The term “direct piezoelectric effect” refers to the internale generation of electrical charge when aa mechanical force is applied to a piezoelectric material, and conversely, the term “converse piezoelectric effect” refers to the internal generation of a mechanical strain when an electrical field is applied to a piezoelectric material. Examples of application of the direct piezoelectric effect include microphones, vibration sensors, accelerometer sensors and the like, and examples of application of the converse piezoelectric effect include speakers, actuators and the like.
Piezoelectric materials have pyroelectricity that is a phenomenon in which a voltage is generated on the surface of the piezoelectric material when the temperature around the piezoelectric material changes. Such materials having pyroelectricity include piezoelectric ceramics and piezoelectric polymers.
Piezoelectric ceramics have been studied since a piezoelectric ceramic material consisting of barium-titanium oxide (BaTiO3) was developed in 1940s and a piezoelectric ceramic material consisting of an oxide of lead-zirconate-titanate (PZT) was developed in 1950s. The piezoelectric ceramics have a hard and dense structure, and thus have advantages in that they are chemically inactive, have tolerance to moisture or temperature variations, and are accurately aligned by mechanical or electrical means. However, these piezoelectric ceramics are disadvantageously brittle, heavy in weight, and inflexible. Particularly, lead contained in the piezoelectric ceramics is harmful to the human body, and thus studies on new piezoelectric ceramics containing no lead have been conducted.
Piezoelectric polymers have been developed since the piezoelectricity of polyvinylidene fluoride (PVDF) was discovered in 1969 by Kawai. Piezoelectric polymers are thin engineering plastics that are easily processed compared to other sensor materials. In addition, these polymers are easily processed in large areas, have flexibility and high impact resistance, are not brittle, are light in weight, and show good sound properties for ultrasonic applications, and good productivity. However, these polymers have disadvantages in that they are used in a limited temperature range and are not suitable for measurement of DC current and the piezoelectric properties thereof are lower than those of piezoelectric ceramics.
Recently, a piezoelectric paper prepared by aligning regenerated cellulose was reported. The piezoelectric paper has piezoelectricity similar to that of existing piezoelectric polymers, and cellulose is biodegradable and biocompatible, and thus does not cause environmental pollution. Due to these advantages, the cellulose piezoelectric paper can be used in the biomedical engineering field related to the human body. In addition, the cellulose piezoelectric paper has excellent heat resistance so as to be capable of resisting high temperatures compared to existing piezoelectric polymers, and thus can be used as a new piezoelectric material. With respect to the cellulose piezoelectric paper, Korean Patent Laid-Open Publication No. 2009-0087280 discloses a piezoelectric paper and a preparation method thereof, the method comprising the steps of: adding sodium hydroxide, DMAc (N,N-dimethylacetamide) or NMMO (N-methylmorpholine-N-oxide) as a solvent to bulk cellulose to make a cellulose solution; subjecting the solution to a spin-coating or casting process to form a thin film comprising cellulose fibers aligned in a specific direction; washing the formed thin film with water to remove the remaining solvent; and disposing an electrode on the formed cellulose thin film.
According to the above document, cellulose paper is bulky paper composed of entangled fibers, and cellulose pulp is dissolved in a solvent such as sodium hydroxide, DMAc or NMMO to make a cellulose solution. Then, when the cellulose solution is spin-coated, the fibers are aligned by the centrifugal force, and when the cellulose solution is extruded, the fibers are aligned in the extrusion direction by mechanical effects such as applied tensile stress. The prepared cellulose film is washed with water to remove the solvent, thereby preparing cellulose paper composed of regenerated paper. When the cellulose paper is mechanically stretched, the cellulose fibers are aligned in the machined direction.
However, the piezoelectric paper prepared as described above has disadvantages in that it has low piezoelectricity and is sensitive to moisture. In addition, piezoelectric polymers or electroactive polymers (EAPs) have advantages in that they are flexible and show a fast response speed and a relatively high displacement, but disadvantages in that they have low piezoelectric properties, require high operating voltages, are prepared at high costs, and particularly cause industrial waste due to lack of biodegradability.
To overcome such disadvantages, the present inventors attempted to use the piezoelectricity of zinc oxide (ZnO) in cellulose. Zinc oxide (ZnO) is a semiconductor material having a wide band gap (3.37 eV) and high electron-hole binding energy (60 meV) and is used in electronic, optical, laser and LED devices, etc. Particularly, zinc oxide having high piezoelectricity can be used in sensors, signal transformers and the like. In addition, zinc oxide is biocompatible and biodegradable, and thus can be used in human- and environment-friendly biomedical engineering systems and green energy production systems. However, a film made of zinc oxide is brittle, and thus much care is required in making products using this film. A zinc oxide film can be fabricated by a sol-gel method or a metal-oxide chemical vapor deposition (MOCVD) method, but it is difficult to fabricate a zinc oxide film on a flexible piezoelectric paper such as cellulose by these methods.
With respect to the application of a zinc oxide film to a cellulose film, the present inventors filed Korean Patent Application No. 2010-0039564, entitled “Cellulose-ZnO piezoelectric paper and preparation method thereof”. According to the document, a cellulose-zinc oxide piezoelectric paper is prepared by adding a wet cellulose film to a solution of zinc nitrate and triethanolamine, reacting the solution with the cellulose film to produce and grow zinc oxide (ZnO) particles on the surface and/or inside of the cellulose film to thereby form a cellulose-zinc oxide composite film, stretching the cellulose-zinc oxide composite film at a predetermined ratio, and drying the stretched cellulose-zinc oxide composite film using a near infrared lamp. However, the cellulose-zinc oxide piezoelectric paper prepared by this method has the disadvantage of low piezoelectric performance, because zinc oxide particles do not strongly adhere to the surface of the cellulose film.